tests/t0107-snapshot.sh: make testscript match updated snapshot behavior

tests/t0107-snapshot.sh: make testscript match updated snapshot behavior

Since cgit now returns a 404 error when the snapshot filename cannot be
resolved to a commit object, the testscript needs to request a valid
snapshot name.

Also, the script assumed that the toplevel directory in the snapshot
would get the name of the repository but it's now named similar to
the requested snapshot.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>